This invention relates to batteries.
Batteries, such as primary alkaline batteries, are commonly used as energy sources. Generally, alkaline batteries include a cathode, an anode, a separator, and an electrolytic solution. The cathode can include, for example, manganese dioxide particles as the active material, carbon particles that enhance the conductivity of the cathode, and a binder. The anode may be, for example, a gel including zinc particles as the active material. The separator is disposed between the cathode and the anode. The electrolytic solution can be, for example, a hydroxide solution that is dispersed throughout the battery.
It is desirable for a primary alkaline battery to have a high discharge capacity. One factor that affects the discharge capacity is the quantity of manganese dioxide in the cathode. As a general rule, the more manganese dioxide in the cathode, the higher the discharge capacity of the battery. But the cathode must also include a sufficient quantity of carbon particles for adequate conductivity. Thus, the quantity of manganese dioxide that can be included in the cathode is limited in part by the quantity of carbon particles required for adequate conductivity.
Typically, the carbon particles used in cathode are composed of graphite. It is known to use a specific type of graphite known as expanded graphite in place of normal graphite particles in the cathode. Expanded graphite is a flake graphite which is chemically treated and heated at a high temperature resulting in the exfoliation of the graphite particles. This produces a graphite with a significantly higher surface area thereby providing better contact with the manganese dioxide particles in the cathode. As a result, a specific quantity of expanded graphite particles provides a higher degree of conductivity to the cathode than the same quantity of non-expanded graphite particles. This in turn means that less graphite and more manganese dioxide can be included in the cathode, thus increasing the discharge capacity of the battery.
The invention relates to using a blend of non-expanded graphite particles and expanded graphite particles in the cathode of primary alkaline batteries. The blend provides the cathode with enhanced conductivity in comparison to a cathode including only non-expanded graphite particles (in the same total quantity), but also is more lubricious and thus easier to process than expanded graphite particles alone. Moreover, because expanded graphite generally is more expensive than natural graphite, the cost of the graphite used in the battery is reduced as compared with cathodes including only expanded graphite.
The blend includes, for example, between 25% and 75% expanded graphite particles by weight, with the remainder being non-expanded graphite particles. Preferably, the blend includes at least 35%, and more preferably at least 45% expanded graphite particles by weight. The expanded graphite particles can have, for example, a particle size of between 15 microns and 40 microns, preferably between 18 microns and 30 microns, and an apparent density of between 0.05 g/cc and 0.10 g/cc, preferably between 0.06 g/cc and 0.08 g/cc. The non-expanded graphite particles can have, for example, a particle size of less than 20 microns, preferably between 2 microns and 12 microns, and more preferably between 5 microns and 9 microns. For purposes of this application, particle size is measured using a Sympatec HELOS analyzer. The expanded graphite particles and non-expanded graphite particles can be natural or synthetic.
Preferably, a battery having a cathode including manganese dioxide and the blend provides at least 95%, preferably at least 97%, and more preferably at least 98%, of the performance in the one watt continuous discharge test (described subsequently), the pulsed regime (described subsequently), and the photo pulse test (described subsequently) as a battery having the same components except that non-expanded graphite has been substituted for a portion of the expanded graphite in the blend. Thus, the cathode including the blend provides substantially the same performance as the cathode including only expanded graphite in the same quantity as the blend, but at reduced cost and higher lubricity.
The cathode blend can have an electrical resistivity, for example, of less than 1 ohm/cm, and preferably less than 0.5 ohm-cm. The cathode can contain, for example, between 3% and 7%, and preferably between 5% and 6%, of the graphite blend by weight.
The invention also relates to a cathode including manganese dioxide and the blend of non-expanded graphite particles and expanded graphite particles.
Other features and advantages of the invention will be apparent from the description of the preferred embodiments thereof and from the claims.